Use of Gaudi in the LVL2 Trigger: The Steering Controller

A proposal is made for a thin interface layer between the Dataflow framework and the Level-2 event selection framework. After consideration of the Dataflow and the Level-2 Trigger requirements, we determine that both frameworks can be interfaced by means of a Finite State Machine, defining a clear boundary between the HLT data collection and the HLT event selection software. A prototype for this interface -- the Steering Controller -- is developed based on LHCb's Gaudi framework, which is also the basis for the ATLAS Event Filter and offline frameworks. The implications of choosing these "off-the-shelf" software components, both in terms of performance and architecture, are the subject of this note.